The instant invention relates to flow metering devices and more particularly to a flow metering device of the type which is operative for determining the flow rate of a fluid in a conduit by measuring the pressure differential produced as the fluid passes through a restriction in the conduit.
The use of flow metering devices of the pressure differential producing type is widely known, and devices of this general type have been heretofore available in a variety of configurations. The U.S. patents to Terrell U.S. Pat. No. 2,868,013; Schroder U.S. Pat. No. 2,863,318; Shaffer U.S. Pat. No. 2,872,810; O'Keeffe U.S. Pat. No. 3,013,432; Halmi U.S. Pat. No. 3,733,901; Halmi U.S. Pat. No. 3,733,902; and Bradham U.S. Pat. No. 4,174,734; and the U.K. patent to Nathan Pat. No. 473,562, are exemplary in this regard and represent the closest piror art to the instant invention of which the applicant is aware. The classical venturi tube is also exemplary of this type of device and for years has found wide acceptance in industry. Generally, the classical venturi tube comprises an elongated shaped tubular member having a substantially straight inlet section, an elongated tapered converging section, a substantially straight throat section of reduced cross section, and a downstream diverging section (energy recovery section). Pressure sensing taps are provided in the inlet and throat sections for sensing the pressure differential produced as the cross-sectional area of the flow is reduced by the tube. In an ideal situation, the energy content of a fluid (the sum of the kinetic and potential energies of the fluid) remains constant as the fluid passes through a venturi tube. However, since the velocity of the fluid must increase as the cross sectional area of the flow is reduced in the throat section of the tube, the kinetic energy of the fluid is increased in the throat section, and therefore the potential energy of the fluid in the throat section is correspondingly reduced. Theoretically, the pressure taps in the inlet and throat sections sense static pressures which are related to the potential energy of the fluid in these sections. Accordingly, the pressure differerential between the inlet and throat sections is related to the difference in the potential energy of the fluid in these sections and inversely related to the difference in the kinetic energy of the fluid in these sections. As a result, in an ideal situation, the rate of the flow of a fluid through a venturi tube can be calculated from the pressure differential between the inlet and throat sections of the tube and the inlet and throat diameters of the tube using an ideal flow equation.
In practice, however, the actual rate of the flow of a fluid through a venturi tube has proven to be somewhat different from the theoretical value calculated in an ideal flow equation. Therefore, in actual practice when high accuracy has been required it has proven to be necessary to multiply the theoretical flow rate by an empirically determined discharge coefficient which must be determined by means of a flow calibration (a physical measurement of the amount of fluid passing through the tube over a specified period of time) in order to establish the actual flow rate of the fluid through the venturi tube. It has been found that relatively high accuracy can be obtained in this manner and that an empirically determined discharge coefficient compensates for the following physical characteristics:
(1) the effects of the Reynolds number as the fluid velocity changes; PA1 (2) the effects of energy losses in the device; PA1 (3) the effects of sheer forces produced by the fluid as it passes the taps causing the formation of vorticies therein; PA1 (4) the effects of tap configurations, including machining irregularities, burrs, edge sharpness, hole size, surface finish, etc; and PA1 (5) the effects of the interior configuration of the venturi tube per se in the areas thereof adjacent the inlet and throat taps.
It will be seen that the above factors can be grouped into two general categories; factors (1) and (2) which relate to the basic configuration of the venturi tube, including the various dimensions thereof, and factors (3) through (5) which relate to the interactions between the fluid and the pressure taps and which therefore vary with each specific venturi tube. Accordingly, while the effects of factors (1) and (2) can be uniformly established for all venturi tubes of a particular dimension and configuration, the effects of factors (3) through (5) vary with each specific venturi tube. While it has been found that the effects of factors (3) through (5) can be reduced by machining venturi tubes with precision tolerances, heretofore it has not been possible to eliminate the effects of these factors. As a result, heretofore it has not been possible to provide highly accurate discharge coefficients with bench calibrations for venturi tubes, and it has been necessary to individually determine discharge coefficients for venturi tubes by making flow calibrations in applications where high accuracy has been required. Heretofore this has been a major problem in the manufacture of venturi tubes and has substantially increased manufacturing costs.
The above factors have also affected other types of flow metering devices and have necessitated individual discharge coefficient determinations for a wide variety of the heretofore available flow metering devices whenever high accuracy has been required. In fact, virtually all of the heretofore available flow metering devices which have relied on pressure readings to determine flow rates have had this problem, primarily because of the interactions between the fluids passing through the tubes and the pressure sensing taps in the tubes.
The instant invention provides a novel flow metering device which is operative for providing accurate flow readings with high energy recovery using bench calibrated discharge coefficients, and therefore the device of the instant invention can be used without requiring flow calibrations for high accuracy applications. Specifically, the flow metering device of the instant invention comprises a shaped tubular member having inlet, converging, and throat sections, and a downstream diverging section for high energy recovery and having pressure sensing taps for sensing the pressure in the inlet and throat sections. In contrast to the heretofore known flow metering devices, however, the device of the instant invention has preferably two annular recesses in the interior thereof, one of the recesses communicating with the inlet section, and the other communicating with the throat section of the device. The pressure sensing taps for the inlet and throat sections are disposed within the recesses, preferably adjacent the respective leading edges of the recesses, and hence the taps only communicate with a fluid passing through the device through the respective recesses. Accordingly, there is virtually no interaction between the moving fluid passing through the device and the pressure sensing taps so that the configurations of the taps are inconsequential. As a result, the pressure sensing taps have very little effect on the discharge coefficient of the device, and really the only factors which influence the discharge coefficient are the effects of the Reynolds number of the fluid as the velocity of the fluid changes and the effects of the energy losses in the device. Since these effects are relatively consistent for all devices of a particular dimension and configuration, a single discharge coefficient can be determined with one flow calibration, and this discharge coefficient can be used to provide bench calibrated discharge coefficients for other devices of the same dimension and configuration. Accordingly, it is seen that by positioning the pressure sensing taps in annular recesses, the effects of the taps on the discharge coefficient of the device can be minimized so that accurate flow readings can be obtained without independently determining discharge coefficients for each device. In addition, since the taps in the inlet and throat sections are static fluid pressures, it is possible to calculate venturi-type adiabatic gas expansion factors for the devices of the instant invention so that they can be used for gases as well as for liquids.
In the preferred embodiment of the device of the instant invention, a third pressure sensing tap is provided in the converging section for sensing the fluid pressure therein. In this regard, while metering devices have heretofore been available, such as the devices commonly referred to as flow tubes which have had converging section taps, the concept of providing three pressure sensing taps, one in the inlet section, one in the converging section, and one in the throat section, provides specific advantages over the prior art devices. When a flow measuring device is embodied with three taps in this manner, two separate differential pressure readings can be monitored to provide improved accuracy in flow readings. Specifically, a first differential pressure between the tap in the inlet section and the tap in the throat section is monitored, this reading being similar to the reading obtained in the flow measuring device when it is embodied with only two taps as hereinabove described. A second differential pressure reading is also observed between the tap in the inlet section and the tap in the converging section, this reading being similar to the type of reading obtained from a conventinal flow tube and being influenced by the dynamic pressure of the fluid in the converging section which causes an aspirating or suction effect on the third tap to produce a magnified differential pressure reading. By observing the relationships between the two differential pressure readings, highly reliable and accurate flow measurements can be determined with the device. Specifically, since a flow reading can be determined by measuring the pressure differential between the inlet and throat taps and applying this reading in a flow equation using a bench calibrated discharge coefficient, the differential observed between the inlet and throat taps can be used to correlate the device for operation with the differential pressure reading observed between the inlet and converging section taps. Accordingly, the device can be operated utilizing the inlet and converging section taps so that a magnified differential pressure reading is observed for increased sensitivity; but because of the unique features of the recessed inlet and throat taps, independent flow calibrations are unnecessary. Further, by monitioring both the pressure differential between the inlet and throat taps and the pressure differential between the inlet and converging section taps, it is possible to detect malfunctions in the device, such as deposit build-ups, etc.; because in most instances a malfunction will cause a change in the interrelation between the two differential pressure readings.
Accordingly, it is a primary object of the instant invention to provide a flow measuring device with high energy recovery wherein the readings produced by pressure sensing taps in the inlet and throat sections of the device are unaffected by tap configurations.
Another object of the instant invention is to provide a flow measuring device which can be bench calibrated with high accuracy.
A further object of the instant invention is to provide a flow measuring device having inlet and throat pressure sensing taps which communicate with the inlet and throat regions of the device through annular recesses, respectively.
A still further object of the instant invention is to provide a flow measuring device having pressure sensing taps in the inlet, converging, and throat sections thereof.
An even still further object of the instant invention is to provide a flow measuring device wherein it is possible to determine when it is malfunctioning without dismantling the device.
While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the appended claims.